1. Field of Invention
The present invention relates to a method of causing an in-plane lattice constant of a donor layer to change.
2. Description of Related Art
Semiconductor light-emitting devices including light emitting diodes (LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavity laser diodes (VCSELs), and edge emitting lasers are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a sapphire, silicon carbide, III-nitride, or other suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The stack often includes one or more n-type layers doped with, for example, Si, formed over the substrate, one or more light emitting layers in an active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region. Electrical contacts are formed on the n- and p-type regions.
FIG. 1 illustrates a composite growth substrate, described in more detail in US 2007/0072324, which is incorporated herein by reference. Substrate 10 includes a host substrate 12, a seed layer 16, and a bonding layer 14 that bonds host 12 to seed 16. Host substrate 12 provides mechanical support to substrate 10 and to the semiconductor device layers 18 grown over substrate 10. Seed layer 16 is the layer on which device layers 18 are grown, thus it must be a material on which III-nitride crystal can nucleate.
As used herein, an “in-plane” lattice constant refers to the actual lattice constant of a semiconductor layer within the device, and a “bulk” lattice constant refers to the lattice constant of relaxed, free-standing material of a given composition. The amount of strain in a layer is defined as |ain-plane−abulk|/abulk. The bulk lattice constant of a ternary or quaternary III-nitride compound AxByCzN may be estimated according to Vegards law, where ax,y,z=x(aAN)+y(aBN)+z(aCN), where a refers to the bulk lattice constants of the binary compounds. AN has a bulk lattice constant of 3.111 Å, InN has a bulk lattice constant of 3.544 Å, and GaN has a bulk lattice constant of 3.1885 Å.
When the seed layer of the composite substrate of FIG. 1 is a III-nitride material, the seed layer is grown strained on the growth substrate, meaning that ain-plane is not equal to abulk. When the seed layer 16 is connected to host substrate 12 and released from the growth substrate, if the connection between seed layer 16 and host substrate 12 is compliant, for example through a bonding layer 14, seed layer 16 may at least partially relax. For example, when a III-nitride device is conventionally grown on Al2O3, the first layer grown on the substrate is generally a GaN buffer layer with an a lattice constant of about 3.19. The GaN buffer layer sets the in-plane lattice constant for all of the device layers grown over the buffer layer, including the light emitting layer which is often InGaN. Since InGaN has a larger bulk lattice constant than GaN, the light emitting layer is strained when grown over a GaN buffer layer. In a composite substrate with an InGaN seed layer, after relaxing, the InGaN seed layer may have a larger in-plane lattice constant than GaN. As such, the in-plane lattice constant of the InGaN seed layer is a closer match than GaN to the bulk lattice constant of the InGaN light emitting layer. The device layers grown over the InGaN seed layer, including the InGaN light emitting layer, will replicate the in-plane lattice constant of the InGaN seed layer. Accordingly, an InGaN light emitting layer grown on a relaxed InGaN seed layer may be less strained than an InGaN light emitting layer grown on a GaN buffer layer.